The synthesis of pyridylalkyl thiols has been conventionally described and reported in many documents and literatures. For example, in the case of synthesis of 2-(4-pyridyl)ethanethiol, there has been practically used the method of reacting 4-vinyl pyridine with thiourea in an ethanol solvent in the presence of p-toluenesulfonic acid to produce an isothiuronium salt, and then converting the thus obtained salt into 2-(4-pyridyl)ethanethiol in aqueous ammonia (for example, refer to “J. Org. Chem.”, 26, 82(1961)). The above method has been continuously improved (for example, refer to Japanese Patent Application Laid-open (KOKAI) Nos. 11-228540(1999) and 11-255748(1999)).
Also, there has been proposed the method of simply producing 2-(4-pyridyl)ethanethiol without isolating the isothiuronium salt by conducting a production reaction of the isothiuronium salt in an aqueous solvent, and then reacting the obtained reaction solution with aqueous ammonia solution (for example, refer to Japanese Patent Application Laid-open (KOKAI) No. 2002-220373).
As a method for producing 4-vinyl pyridine as one of the starting materials of the above 2-(4-pyridyl)ethanethiol, there is generally known the method of subjecting γ-picoline and formaldehyde to methylolation reaction to produce 2-(4-pyridyl)ethanol, and then subjecting the thus produced 2-(4-pyridyl)ethanol to dehydration reaction (for example, refer to Japanese Patent Application Laid-open (KOKAI) No. 53-144577(1978)). The thus produced 4-vinyl pyridine contains several kinds of impurities even after being purified, for example, by distillation. These impurities include γ-picoline as an unreacted raw material, as well as various by-products such as ethyl pyridine, isopropenyl pyridine, propenyl pyridine and methylvinyl pyridines having methyl and vinyl groups bonded to a pyridine skeleton thereof.
In addition to the above methods, there are also known the following methods. That is, there have been proposed the method of using as a sulfur-containing compound, a thioacetic acid instead of urea, and reacting the thioacetic acid with vinyl pyridine to produce pyridylethylthioacetate as a pyridylethylthio compound (for example, refer to U.S. Pat. Nos. 6,534,686 and 6,620,939), and the method of reacting vinyl pyridine with hydrogen sulfide as the sulfur-containing compound to produce pyridylethanethiol as the pyridylethylthio compound (for example, refer to U.S. Pat. No. 6,667,402). Meanwhile, the above pyridylethylthioacetate is decomposed in the presence of an acid and readily converted into pyridylethanethiol. However, the pyridylethylthioacetate as a pyridylethanethiol derivative having a mercapto group protected by an acetyl group may also be directly as a modifier for modifying a catalyst used upon producing bisphenol A by condensation of phenol and acetone.